1. Field of the Invention
The present invention relates to a mask conveniently used for seal coating of a semiconductor element and a method of fabricating a sealed type semiconductor device using the mask.
2. Discussion of Background Information
A semiconductor device is fabricated with a semiconductor element mounted on a wiring board. The semiconductor element is electrically connected to the wiring board with bonding wire or with bumps formed on the semiconductor. After connection to the wiring board, the semiconductor element is sealed with a sealing material.
In the sealing process, as shown in FIG. 11(a) a semiconductor element 2 is mounted on a wiring board 1. The semiconductor element 2 is electrically connected to a circuit (not shown) of the wiring board 1 by bonding wires W. After this connection, a coating mask 3', with an opening 31' located at the mounting position of semiconductor element 2 on wiring board 1, is overlaid on wiring board 1 such that semiconductor element 2 is located inside opening 31'. A sealing material 4, supplied in advance on the mask 3', is then forced in with a squeegee 5, whereby sealing material 4 is coated as a pattern on the front surface of wiring board 1 through mask opening 31'.
FIG. 11(b) shows a semiconductor device obtained after removing mask 3' from wiring board 1. When mask 3' is removed from wiring board 1, the sealing material 4, which has a viscosity, attaches to the inner peripheral surface 311' of opening 31' as shown in FIG. 12(a). Under this condition, mask 3' is removed. The sealing material 4 develops a stringiness. The stringy sealing material 4 curls onto the back suite of mask 3' as shown in FIG. 12(b). This curled portion 41 is pressed and forms a pattern on the surface of the wiring board 1 as shown in FIG. 12(c) the next time when mask 3' is used. The amount of this curled portion 41 is variable according to the position of the back peripheral edge of opening 31', and therefore the spread 42 on the front surface of the wiring board assumes an irregular pattern as shown in FIG. 12(d).
For the purpose of preventing this phenomenon, Japanese Patent document 6-95594 discloses a mask 3" formed with an annular protrusion 32" in linear contact along the whole periphery of the edge of the back of an opening 31" as shown in FIG. 13.
Also, Japanese Patent document 3-181142 discloses a space portion 33" formed outside of the annular protrusion 32" for cutting off the sealing material as shown in FIG. 14. Specifically, by forming space portion 33" on the back of the mask, the sealing material is prevented from curling around to the outside of annular protrusion 32".
FIG. 16(a) shows the method disclosed in Japanese Patent document 6-95594. When mask 3" is removed from wiring board 1 after sealing, the sealing material 4 remains attached in drips at the forward end of the annular protrusion 32". Thus, when mask 3" is set on a new wiring board 1 at the next coating session, the sealing material 4 in drips is pressed against new wiring board 1 and spreads over the surface of wiring board 1 as shown in FIG. 16(b). The size of the drips, however, is varied from one point to another of the annular protrusion (in the case where the annular protrusion is in the form of a rectangular frame, for example, the drips on the side portions are thicker than those at the corners). Therefore, the spread described above is inconsistent and irregular, and not uniform. In other words, a sediment deposit appearance develops. If the sealing material is forced in and the mask 3" is removed from wiring board 1 under this condition, the foot of the sealing material thus forced in extends over the entire irregular spread that has already expanded over the front surface of the wiring board, with the result that the foot of the sealing layer assumes an irregular jagged form and cannot be finished in an attractive manner. The recent trend of semiconductor elements is toward a higher density package, and the sealing area is strictly defined. An irregular jagged foot of the sealing layer, therefore, is liable to cause a conduction failure. For this reason, it is desirable that the drips at the forward end of the annular protrusion are prevented from spreading over the surface of the wiring board.